1. Field of the Invention
This invention relates to a method for manufacturing a golf ball comprising a core and a cover of one or more layers around the core, the method being capable of forming the cover layer uniformly to a significantly reduced thickness.
2. Background Art
In golf balls comprising a core and a cover of one or more layers around the core, the materials and thicknesses of the core and the cover and combinations thereof are selected as appropriate in accordance with the various characteristics required as competition balls and the favor of players.
Since the core makes a major contribution to the rebound of golf balls, the cover layer is advantageously formed thinner if flight performance is of more interest.
In the prior art, solid golf balls comprising a spherical core of an elastomer, typically rubber, enclosed with a resin cover are manufactured in several ways. For example, one typical method uses a mold consisting of a pair of upper and lower split mold halves which define a spherical cavity when mated, have a plurality of dimple-shaping protrusions on their inner wall and are provided with a plurality of vertically extending support pins. An elastic core is vertically held in place within the cavity by means of the support pins. A cover material is injected around the core to form a cover.
However, the injection molding technique is difficult to form a cover uniformly to a thickness of 1 mm or less. The reason is described below. Typically four to eight gates are disposed in the parting plane between the mold halves corresponding to the equator of the cavity. The injection pressure of the cover material from these gates is very high along the equator. The support pins are retracted to be flush with the mold inner wall immediately before the end of cover material injection. After the retraction of support pins, the injection pressure of the cover material causes the core to be deformed into an elliptic or rugby ball shape elongated in polar directions, and the cover is molded on the core in the deformed state. As a result, the golf ball as molded has the cover which is thicker near the equator and thinner near the opposite poles, and in extreme cases, so thin that the dimple bottom is in direct contact with the core surface. The resulting golf ball lacks uniformity substantially. Thin cover portions are formed near the opposite poles because the space between the core surface and the mold inner wall is narrow, which can obstruct the flow of the cover material during injection molding. This results in that weld marks are left on the cover surface after the molding, detracting from the outer appearance. Thus the above-described method is difficult to manufacture golf balls having a cover which is as thin as 1 mm or less.
In another known method, as also described in JP-A 61-199872, a spherical elastic core is encased in a pair of half shells which have been separately molded from resinous cover material. The assembly is placed in a spherical cavity defined by a pair of upper and lower mold halves having a plurality of dimple-shaping protrusions on their inner wall, where compression molding is effected by applying heat and pressure.
When the cover is molded in this way, the air trapped between the shells on the core and the mold inner wall and any extra cover material (often amounting to so much) run out from the parting plane between the mold halves. Fins are formed by curing of the run-out cover material and integrated with the golf ball as molded. It is cumbersome to trim the fins and polish for finishing. Additionally, the trapped air is not completely expelled. Residual air trap causes defective outer appearance, resulting in balls to be rejected.
There remains a need for golf balls having a cover layer whose thickness is very thin and uniform and which has an aesthetic outer appearance.